“ZARONIA stands for the South African Rand Overnight Index Average. It is a benchmark interest rate published daily by the South African Reserve Bank (SARB), calculated as the weighted average of actual, unsecured overnight loans between commercial banks.” – ZARONIA – Finance

Shifts in interest rate benchmarks reshape how banks fund themselves, how corporates borrow, and how investors price risk across the financial system. The move towards transaction-based overnight reference rates embodies a broader post-crisis push for robustness, transparency, and regulatory alignment, and South Africa’s adoption of a new overnight rand benchmark sits squarely within that global reform agenda.^2,4,9,17 The change influences everything from interbank liquidity management to the legal drafting of loan agreements, and it does so by replacing judgement-heavy, forward-looking benchmarks with rates grounded in observed overnight funding costs.^4,10,19

The underlying problem with legacy benchmarks

Legacy interbank benchmarks in South Africa, most notably the Johannesburg Interbank Average Rate (JIBAR), were built around indicative quotations for term unsecured lending rather than deep, liquid transaction data.^4,10,19 As wholesale unsecured term markets shrank over time, fewer underlying trades meant the benchmark increasingly relied on expert judgement, exposing it to both manipulation risk and representativeness concerns.^4,9,19 International reform of IBOR-type rates, triggered by misconduct scandals and structural shifts in bank funding markets, highlighted these weaknesses and led regulators to question whether such benchmarks could continue to serve as reliable references for the valuation and hedging of trillions of rand in financial contracts.^4,9,17

In practice, reliance on thin markets introduces multiple vulnerabilities. Where the underlying data set is small, extreme quotes or idiosyncratic funding pressures at individual banks can skew the benchmark relative to wider market conditions.^4,9 More fundamentally, a benchmark that is not clearly anchored in observable trading fails basic tests of transparency and may be difficult to defend under evolving regulatory standards such as benchmark regulation and conduct guidelines.^4,19 These concerns are particularly acute when the benchmark underpins retail and corporate lending, long-dated derivatives, and capital markets instruments, where even modest misalignment between the reference rate and actual funding costs can have significant distributional consequences over time.

Benchmark reform and the pivot to transaction-based overnight rates

The South African Reserve Bank (SARB), working with the Market Practitioners Group (MPG), launched a comprehensive interest rate benchmark reform programme to address these weaknesses and align local practice with international moves towards nearly risk-free reference rates.^2,3,6,9 The reform introduced a suite of new overnight benchmarks, both unsecured and secured, and identified a transaction-based rand overnight rate as the preferred successor to JIBAR for many applications.^2,3,6,9 The policy goal is clear: reference rates should be based on broad, representative sets of actual trades; they should be robust under stress; and their methodologies should be clearly documented, with governance frameworks that reduce scope for discretion and manipulation.^2,3,6,19

Overnight rates are well suited to these objectives because the overnight unsecured deposit and interbank lending markets tend to be deeper and more active than longer-term unsecured funding markets.^2,9,17 Daily turnover in overnight call deposits provides a rich data set to extract a representative benchmark for banks’ marginal funding costs, while the short maturity reduces term credit and liquidity premia, making the resulting rate closer to a risk-free or near risk-free benchmark.^9,17 From a modelling perspective, using an overnight reference rate as the anchor for discounting and valuation also reduces structural biases associated with predicting term rates months or years ahead, since compounded overnight rates are built from realised daily outcomes rather than forward-looking quotes.^10,17,19

Methodological substance: how the rate is constructed

The benchmark is calculated using data on unsecured overnight deposits and loans between commercial banks operating in the South African rand market.^2,11,13,14 Eligible transactions include wholesale call deposits and interbank overnight lending above specified size thresholds, executed on South African business days and reported to the administrator’s infrastructure.^2,11,13 Each qualifying trade contributes both a volume and a rate, forming the basis for a volume-weighted average of overnight funding costs.^2,13,17 This structure ensures that larger trades carry more influence in the calculation, reflecting their greater economic significance and the fact that they typically occur at rates that clear the core of the overnight market.^2,13,17

To enhance robustness, the administrator applies a trimming mechanism to the distribution of transaction rates before computing the mean.^2,11,13,17 Conceptually, if reported overnight rates are denoted r_i with associated volumes v_i, the calculation begins by ordering the transactions and removing a small proportion of volume at the extremes of the rate distribution, thereby excluding outliers that may reflect idiosyncratic credit situations or data anomalies.^2,13,17 The benchmark is then given by the trimmed, volume-weighted mean

\text{ZARONIA} = \frac{\sum_{i \in T} v_i r_i}{\sum_{i \in T} v_i}

where T is the set of transactions remaining after trimming.^2,13,17 All symbols here appear only inside the LaTeX block, as required. This formulation produces a more stable and representative measure of overnight funding costs than a simple untrimmed average, particularly during periods when a small number of trades occur at unusually high or low rates.^2,13

The SARB, as administrator, publishes the benchmark each South African business day, typically at 10:00, and retains the ability to correct errors by republishing before midday.^2,11 Alongside the overnight rate itself, the SARB now also publishes compounded period averages derived from daily observations, providing standard tenors such as 1-week, 1-month, and 3-month backward-looking term rates for use in contracts and risk management.¹⁷ These compounded figures are calculated using the standard daily compounding formula applied to the overnight series, ensuring consistency with global risk-free rate conventions.¹⁷

Practical meaning in funding, lending, and derivatives

In practical terms, the benchmark is intended to be a near risk-free reference rate for the South African rand money market.^4,9,17 Because it is based on unsecured overnight interbank transactions, it captures the marginal cost at which banks obtain wholesale rand funding overnight, net of minimal credit and liquidity premia.^2,9,14 This makes it suitable as a foundational rate for a broad range of financial products, including floating-rate loans, bonds, securitisations, and derivatives that currently reference JIBAR.^4,10,17 By switching to a transaction-based overnight rate, market participants gain a benchmark that more closely tracks actual funding conditions, improving the alignment between contractual cash flows and underlying economics.^4,10,19

For banks, the benchmark becomes a key input into treasury management and liquidity planning. Overnight funding desks compare their own borrowing and lending rates to the benchmark to assess whether they are paying or receiving above-market levels, and they can use the rate as a reference point when pricing overnight call accounts, commercial paper, and short-term instruments.^2,4,9 For corporates, the benchmark will increasingly underpin loan margins and the pricing of revolving credit facilities, often via compounded overnight conventions that replace traditional fixed term JIBAR settings.^4,10,17 Investors in money market funds and floating-rate notes benefit from the fact that interest receipts linked to a transaction-based benchmark may more accurately reflect prevailing money market conditions instead of legacy term quotes.^9,10,17

In the derivatives market, the benchmark is expected to become the primary discounting and floating leg reference rate for rand interest rate swaps, overnight index swaps, and related instruments.^4,17,20 Transitioning swap books from JIBAR to an overnight risk-free rate affects valuations, hedge effectiveness, and collateral interest calculations, particularly where discounting and collateral remuneration are aligned to the new rate.^4,17,19 The proliferation of derivatives referencing the overnight benchmark will also enable the construction of forward-looking term rates, such as Term ZARONIA, derived from traded futures and swaps markets rather than bank quotes.²² This layered structure mirrors developments in other jurisdictions, where overnight risk-free rates anchor valuation while term rates derived from them facilitate operational simplicity in loan markets.

Mathematical specification and parameter interpretation

Understanding the benchmark fully requires situating it within the broader structure of risk-free rate mathematics. In continuous-time modelling of interest rates, an overnight risk-free rate process r_t often serves as the short rate in affine or Heath-Jarrow-Morton-type frameworks.^17,19 Discount factors for cash flows at time T are given by

D(0,T) = \exp\left(-\int_0^T r_s \, ds\right)

where r_s is interpreted as the instantaneous overnight rate, approximated in practice by the realised daily benchmark.^17,19 Under risk-neutral pricing, the dynamics of r_t might be specified by a stochastic differential equation, for example a one-factor mean-reverting process

dr_t = a(b - r_t) \, dt + \sigma \, dW_t

with a the speed of mean reversion, b the long-run mean, \sigma the volatility, and W_t a Brownian motion.^17,19 While the benchmark itself is an observed series rather than a model output, such specifications are used to value derivatives and manage risk in markets referencing the rate.

In applied pricing of compounded overnight cash flows, the daily benchmark observations r_i over a period i = 1, \dots, n are used to compute a compounded rate R via

R = \left(\prod_{i=1}^{n} \left(1 + \frac{r_i \Delta t_i}{360}\right)\right) - 1

where \Delta t_i is the day count fraction for day i.¹⁷ This backward-looking rate then determines the interest payment on a notional N over the accrual period, with interest I = N R.¹⁷ Each parameter (overnight rate, day count fraction, accrual period) is operationally specified in benchmark conventions, and the compounded rate mirrors global methodologies used for risk-free rate-based loans and swaps.¹⁷

Schools of thought: overnight risk-free rates versus term benchmarks

There are two broad schools of thought in contemporary benchmark design. One group emphasises overnight risk-free rates as the single source of truth for discounting and valuation, arguing that using backward-looking compounded averages is operationally manageable and conceptually cleaner than relying on forward-looking term quotes.^17,18,19 In this view, a transaction-based overnight rate should be the primary benchmark, with all term structures constructed either from compounding or from derivatives markets referencing that overnight rate. Advocates cite transparency, robustness, and reduced manipulation risk as decisive advantages.^4,9,19

A second school accepts the primacy of overnight risk-free rates for valuation but stresses the practical benefits of forward-looking term rates, particularly in loan markets and treasury operations.^10,17,22 For many borrowers, knowing the applicable interest rate at the start of an accrual period simplifies budgeting, approvals, and operational workflows; backward-looking compounded rates, by contrast, are only known at the end of the period and can complicate cash management.^10,17 This camp therefore backs the development of forward-looking term rates derived from overnight benchmarks, such as Term ZARONIA, which seek to preserve operational convenience while anchoring the term structure in a transparent, transaction-based overnight market.²²

The tension between these approaches plays out in contractual choices and regulatory signalling. Supervisors and central banks tend to favour overnight risk-free rates as fundamental benchmarks and caution against over-reliance on forward-looking term rates where derivatives markets are thin.^2,3,19,22 Market participants, however, often push for pragmatic solutions that balance robustness with usability, especially in sectors where systems and processes are built around known-in-advance term rates.^10,17,22 The resulting compromise typically involves a hierarchy: overnight risk-free rates for discounting and complex instruments, compounded averages for many loans and notes, and forward-looking term rates reserved for cases where derivative liquidity can support a robust benchmark.

Debates around risk-free status and representativeness

Despite being widely described as near risk-free, unsecured overnight interbank benchmarks are not literally free of credit and liquidity risk.^4,9,14 Each transaction reflects the perceived credit quality of the borrowing bank, expectations about central bank policy, and temporary liquidity conditions in the money market.^2,9 In stress episodes, overnight unsecured rates can rise sharply above policy rates and secured funding costs, revealing the presence of a non-trivial risk premium.⁹ Some commentators therefore argue that secured overnight funding benchmarks, such as repo-based rates, offer a purer measure of the risk-free rate.^9,18

Proponents of unsecured overnight benchmarks respond that the residual credit and liquidity premia at overnight maturities are modest in normal conditions and that unsecured rates better reflect the actual marginal funding costs of banks, which is what many contracts implicitly intend to reference.^4,9,17 They also point out that unsecured overnight markets remain central to liquidity management, whereas secured markets may be dominated by collateral and regulatory constraints that introduce their own distortions.^9,18 In South Africa’s case, the choice to build a key benchmark on unsecured overnight deposits reflects both market structure and a desire to capture a rate that is representative of bank funding conditions rather than purely theoretical risk-free levels.^2,9

Representativeness raises a further debate: how wide must the underlying market be for a benchmark to be considered robust? Supporters of the new overnight benchmark note that the volume of overnight unsecured deposits and interbank loans in the rand market is sufficient to support a trimmed, volume-weighted average that is not unduly influenced by a handful of trades.^2,9,13 Critics worry that, in quieter periods, the number of transactions could fall, potentially increasing sensitivity to idiosyncratic trades and making the trimming methodology more consequential.^13,19 The administrator’s transparency about thresholds, trimming parameters, and contingency policies is therefore central to confidence in the rate.^2,3,11,13

Transition from JIBAR and contractual implications

The transition away from JIBAR towards the new overnight benchmark is phased but time-bound. The SARB and MPG have confirmed that JIBAR will be permanently discontinued after its final publication on 31 December 2026, with a key interim milestone often described as the “No New JIBAR” date in 2026, after which new contracts may not reference JIBAR except in limited cases.^4,8,10,12 Financial institutions are expected to stop writing new JIBAR-linked products and to begin actively transitioning existing portfolios to the new benchmark or suitable alternatives well ahead of cessation.^4,8,10,16

Contractually, this transition is complex. Any agreement that references “JIBAR + margin” must either rely on pre-agreed fallback language pointing to a successor rate or be amended to replace the benchmark with the new overnight rate, potentially with an adjustment spread to address historical differences between the two.^7,8,10,19 Fallback clauses vary widely; some are mechanical and designate a successor benchmark or committee decision, while others simply call for commercial renegotiation if the original rate ceases.^7,8,19 Legal teams therefore need to identify impacted contracts, interpret their fallbacks, and engage counterparties where necessary to avoid disputes or unintended economic shifts when legacy benchmarks stop publishing.^7,8,10

Operationally, moving from forward-looking term settings to backward-looking compounded overnight calculations requires system changes. Treasury and finance teams must adjust the timing of rate determination, approval workflows, and cash positioning so that interest amounts based on compounded overnight benchmarks can be processed without delays.^7,10,17 Where hedges exist, they must be transitioned in a coordinated fashion with the underlying funding positions to preserve hedge effectiveness and avoid basis risk between funding and derivative legs.^7,8,17 Regulatory guidance from the Financial Sector Conduct Authority and Prudential Authority has emphasised the need for orderly, well-governed transition programmes that avoid cliff-edge risks at JIBAR cessation.^8,12,19

Why the benchmark still matters and strategic considerations

The importance of the new overnight benchmark extends beyond technical interest rate modelling. It is a cornerstone in the credibility of South Africa’s financial architecture, influencing perceptions of market integrity and regulatory competence. A transparent, transaction-based benchmark supports confidence that pricing in loans, bonds, and derivatives is grounded in actual market behaviour rather than opaque judgement.^4,9,19 This, in turn, helps align South Africa’s markets with global investors’ expectations, facilitating cross-border capital flows and support for local-currency issuance.^4,19

Strategically, adoption of the benchmark gives banks and corporates a clearer lens on liquidity conditions and policy transmission. Because the rate reflects the cost of overnight wholesale funds, movements relative to the policy rate and other benchmarks can reveal shifts in funding stress, risk appetite, or central bank operations.^2,9,23 For risk managers, this makes the benchmark a valuable indicator of short-term market dynamics and a key variable in stress testing and scenario analysis. For product designers, it offers a robust building block for new instruments that can withstand scrutiny under evolving benchmark regulations.^2,3,19

The benchmark also matters because the transition is not a one-off event; governance and methodology will continue to evolve as markets develop. Questions such as the appropriate trimming level, the future of forward-looking term rates, and the interaction between unsecured and secured benchmarks will remain live policy and market issues.^3,6,9,22 As the derivatives market referencing the overnight rate deepens, the ability to construct multi-tenor term structures from it will expand, potentially changing how loans, securitisations, and structured products are designed. Market participants that understand the underlying mechanisms and debates are better positioned to influence these developments rather than simply reacting to them.

Ultimately, the benchmark’s durability will rest on continued representativeness, clear governance, and widespread market adoption. If transaction volumes remain healthy, methodologies stay transparent, and contractual frameworks are updated thoughtfully, the rate can serve for decades as a reliable anchor for rand-denominated finance. For practitioners across treasury, legal, risk, and product functions, engaging seriously with its substance is therefore not optional: it is a prerequisite for navigating an evolving interest rate landscape without missteps.

 

References

1. South African Rand Overnight Index Average (ZARONIA) – Investec – https://www.investec.com/en_za/zaronia/south-african-rand-overnight-index-average-zaronia.html

2. ZARONIA interest rate benchmark – South African Reserve Bank – 2022-10-31 – https://www.resbank.co.za/en/home/what-we-do/financial-markets/south-african-overnight-index-average

3. ZARONIA methodology and policies – South African Reserve Bank – https://www.resbank.co.za/en/home/what-we-do/financial-markets/south-african-overnight-index-average/ZARONIA-methodology-and-policies

4. ZARONIA | Nedbank CIB – https://cib.nedbank.co.za/solutions/markets/zaronia.html

5. South Africa Rand Overnight Index Average ZARONIA – 2015-05-28 – https://tradingeconomics.com/south-africa/interbank-rate

6. SARB publishes interest rate benchmark report | SAnews – 2021-12-01 – https://www.sanews.gov.za/south-africa/sarb-publishes-interest-rate-benchmark-report

7. JIBAR is Ending: What ZARONIA Is, and What to Do Now – LinkedIn – 2026-02-18 – https://www.linkedin.com/pulse/jibar-ending-plain-language-guide-what-changes-zaronia-ravie-govender-yyjpf

8. Rand Rate Reform: Transitioning to ZARONIA – Standard Bank Group – 2025-12-03 – https://www.standardbank.com/sbg/standard-bank-group/our-group/our-business/rand-rate-reform

9. Interest rate benchmarks for SA – Codera Analytics – https://codera.co.za/interest-rate-benchmarks-for-sa/

10. Important information regarding the transition from JIBAR to ZARONIA – 2025-04-09 – https://www.oldmutualinvest.com/institutional/knowledge-room/insights/important-information-regarding-the-transition-from-jibar-to-zaronia/

11. Reserve Bank publishes South African Rand Overnight Index Average – 2022-11-03 – https://www.gov.za/news/media-statements/reserve-bank-publishes-south-african-rand-overnight-index-average-03-nov-2022

12. What is changing when? – RMB – https://www.rmb.co.za/page/what-is-changing-when

13. [PDF] Frequently Asked Question on understanding – ZARONIA – https://saicawebprstorage.blob.core.windows.net/uploads/FAQ-on-understanding-ZARONIA.pdf

14. ZARONIA (The South African Rand Overnight Index Average) – 2025-05-20 – https://cbonds.com/indexes/191013/

15. Historical data: South African Benchmark Overnight Rate – 2025-02-03 – https://www.reservebank.co.za/en/home/publications/publication-detail-pages/Financial-Markets/markets-data/SABOR-Historical-Data/2024/historical-data-south-african-benchmark-overnight-rate-december-2024

16. Preparing for the ZARONIA Transition: Key Milestones … – Tradeweb – 2026-04-17 – https://www.tradeweb.com/newsroom/media-center/insights/blog/preparing-for-the-zaronia-transition-key-milestones-and-what-comes-next/

17. [PDF] Transition to South African Rand Overnight Index Average FAQs – https://cdn.tradeweb.com/sites/ZARONIA/assets/pdf/Tradeweb_EM_Risk-Free_Reference_Rate_Transition_ZARONIA_-_FAQs_Final_Sept_2025.pdf

18. OFR STFM – Federal Reserve Bank of New York reference rates – https://www.financialresearch.gov/short-term-funding-monitor/market-digests/rates/chart-17/

19. South Africa: The Dawn of ZARONIA – A new era for South Africa’s … – 2025-07-08 – https://insightplus.bakermckenzie.com/bm/banking-finance_1/south-africa-the-dawn-of-zaronia-a-new-era-for-south-africas-financial-legal-framework

20. ZARONIA Resource Centre – Tradeweb – https://cdn.tradeweb.com/sites/ZARONIA/index.html

21. Publication of a new interest rate benchmark, the South African … – 2022-11-03 – https://www.polity.org.za/article/publication-of-a-new-interest-rate-benchmark-the-south-african-overnight-index-2022-11-03

22. South Africa: Moving to Term ZARONIA – What it is, how it differs … – 2025-11-05 – https://bowmanslaw.com/insights/south-africa-term-zaronia-explained-what-it-is-how-it-differs-from-compounded-zaronia-and-why-it-matters-now/

23. Current Market Rates – South African Reserve Bank – https://www.resbank.co.za/en/home/what-we-do/statistics/key-statistics/current-market-rates

24. SA Reserve Bank has commenced publishing the South African … – 2022-11-03 – https://www.facebook.com/SaReserveBank/posts/sa-reserve-bank-has-commenced-publishing-the-south-african-rand-overnight-index-/5379080592217879/